This invention relates to high-voltage semiconductor switching devices and more particularly to a structure for reducing the fall time in high-voltage transistors.
Bipolar semiconductor devices in general, have turn-off time limited by stored charge. High voltage devices thus turn-off particularly slowly because the relatively thick lightly-doped regions required for high-voltage capability are very susceptible to charge storage. While storage time per se may not be particularly important in a high-voltage, low frequency application, stored charge also leads to long fall times, which engender excessive power dissipation.
In a high-voltage transistor, for example, it is known to decrease this fall-time by providing narrow emitters, so that stored charge can be removed rapidly laterally from base and collector regions. Such an approach however conflicts with the requirement for large current handling capability in the one condition. Similarly, lifetime reduction to minimize the stored charge is in direct conflict with the requirement for high current gain in the on condition.